Excessive heating of the coils of a linear motor causes an increase in the resistance of the coils, exacerbating the heat problem and reducing the performance of the motor. In addition, this heat is carried away to the outside air and often to the rest of the machine in which the motor is utilized. Heat changes the index of refraction of air and consequently reduces the accuracy of laser interferometers and other optical systems. In addition, the heat causes thermal expansion of machine components, resulting in inaccuracy of precision mechanical systems.
Most commercially available linear motors are not actively cooled. Typically the coils are potted in a moderately conductive epoxy and the motor is cooled through convection into the surrounding air. Trilogy Systems provides an option to their motor where cooling fluid is circulated through a metal mounting bracket of the coil assembly. Because this bracket is mounted only to the top of the motor, not all of the heat is carried away from the motor and a significant portion of it is still convected into the surrounding environment.
U.S. Pat. No. 4,749,921 issued to Anwar Chitayat describes a linear motor. Included is a concept for cooling the linear motor coils. FIG. 8 in this patent shows a system of hollow tubes that are potted with the coil assembly. Coolant can flow through these tubes providing cooling. In U.S. Pat. No. 4,625,132 also issued to Anwar Chitayat, a controlled flow of cooling gas is directed between the motor stator and the moving element with flexible seals on each arm of a U-shaped channel mount a wound stator. In another Chitayat U.S. Pat. No. 4,839,545 an armature of a linear motor is cooled by coolant flowing through a lower serpentine channel in thermal contact with laminations of the motor armature.
U.S. Pat. No. 4,906,878 discloses a linear motor with cross-flow passageways or tubes connecting between inlet and outlet manifolds to remove heat from the motor coils. U.S. Pat. No. 4,916,340 utilizes heat insulating materials with a cooling medium (water) flowing through passageways on coil supporting members. U.S. Pat. No. 5,073,734 discloses a coolant for cooling between linear motor spacers and a screen support having cooling fins.
Yaskawa Japan Laid Open Application Heisei 8-168229 provides a linear motor that is enclosed in a stainless steel can (housing). This can has a small gap along the outside of the coils, which enables (not disclosed) coolant to be forced along the gap between the can and the coils to provide cooling of the motor. Yaskawa Utility Model Application Heisei 5-45102 includes a coil bobbin with a cooling path inside the bobbin.
Typical linear motors that are not cooled have inefficient motor operation due to increased coil resistance with temperature, heating of surrounding air, and heating of surrounding machine elements as discussed above. Motors that are only cooled through the mounting bracket do not provide direct cooling of the coils and suffer from the same disadvantages. The cooled motor of U.S. Pat. No. 4,749,921 and others of the above patents require cooling passages to be created within the coil assembly. This is difficult and can typically only be done by wrapping the coils with tubing and encapsulating the assembly in an epoxy. It also does not completely isolate the motor from the outside air because the cooling tubes do not completely enclose the coils. The Yaskawa disclosures include cooling arrangements which cannot be completely adapted to all motor configurations. In addition, both rely on an exterior thermal insulation or an exterior can (13 and 29, respectively) that may be difficult to fabricate. In both Yaskawa disclosures the cooling is on the inside of the bobbin or inside the can; coil heat may be transferred directly from the coil outer surfaces to the outside environment resulting in detriment to the machine in which the motor is being utilized.